Watched my colleague play it here at work. Level design has its ups and downs...strange arbitrary use of slow motion, abuse of player blocking volumes, instantly killing the player when they veer off the path, terribly confusing objectives in the Battle of Khe Sanh. IGN ripped them a new one about that mission:
http://xbox360.ign.com/articles/113/1133005p1.html
Overall, not bad though...a lot of variety and an interesting story. Black Ops is worth it, as usual, for the whole package not just the SP campaign.